The use of alkoxy-functional silanes for the crosslinking of polymers, especially for the production of pipes, foams, wires and cables, and heat-shrinkable tubing, results in the release of alcohol upon the hydrolysis of the silane. This alcohol is typically methanol or ethanol, and can pose environmental, health, and safety concerns. As the emission of volatile organic compounds (VOCs) becomes more tightly regulated, compounders and pipe, foam, tubing, wire and cable producers are many times forced to reduce production, install recovery or remediation equipment, or use special engineering controls to meet new, stricter emission limits as well as mitigate explosions or flammability hazards. As an example, potable water pipe manufactures are facing increasing limitations on allowable methanol content in as-produced pipes. These producers need a more cost-effective way to reduce the presence and emission of VOCs from their processes that employ conventional silanes.
Silanes are commonly employed as crosslinkers for the production of PEX-b pipe (silane crosslinked polyethylene), wire coatings, insulation jacketing for low- and medium-voltage cables, foams for insulation, and heat shrinkable products such as tubing. The silane is typically used in conjunction with a peroxide, which is used to effect grafting of the silane onto the polymer it will be crosslinking. Other additives, such as anti-oxidants, metal de-activators, condensation catalysts and so forth can also be included.
The most commonly employed silanes are vinyl-functional silanes with vinyltrimethoxy silane being the most prevalent. The production of crosslinked polymers involves grafting the silane onto the polymer and hydrolysis and condensation of the silane to provide crosslinked polymers. The grafting reaction is typically performed in a single screw extruder, while the hydrolysis/condensation reaction can be done under a variety of conditions, including exposure to moisture under ambient conditions, exposure to hot water via submersion of the grafted resin, or exposure to steam. In the production of potable water pipes, for example, hot water is circulated through the extruded pipe to complete the crosslinking. The circulation of hot water for an extended period also helps to remove the by-product methanol that is generated during the crosslinking. It is necessary to circulate water through the pipes until the methanol levels have decreased below allowable limits.
Silane crosslinking of polymers can lead to many improvements in chemical resistance, abrasion resistance, high temperature deformation resistance, wet and dry electrical properties, scratch resistance, tensile strength, flexural strength, creep, stress rupture properties, memory effect, impact strength, resistance to ageing, reduced drip phenomena, and other mechanical properties.